Duval's Triangle Fault Regions
Each region of Duval's Triangle corresponds to a specific fault type identified from the relative concentrations of methane (CH4), acetylene (C2H2), and ethylene (C2H4) dissolved in transformer oil.
Quick Reference Table
| Label | Fault Type | Description |
|---|---|---|
| PD | Corona / Partial Discharges | Low-energy discharges in gas-filled cavities — early sign of insulation aging |
| D1 | Low-Energy Discharges | Sparking or arcing from poor connections or floating components |
| D2 | High-Energy Discharges | Sustained arcing that produces large gas volumes and rapid degradation |
| DT | Mixed Electrical & Thermal | Combined discharge and thermal decomposition |
| T1 | Thermal Fault < 300°C | Low-temperature overheating from overload or poor cooling |
| T2 | Thermal Fault 300–700°C | Medium-temperature hot spots in core or windings |
| T3 | Thermal Fault > 700°C | High-temperature overheating from sustained arcing or severe faults |
Detailed Region Descriptions
PD — Corona / Partial Discharges
Partial discharges are low-energy electrical discharges that occur within gas-filled cavities in the transformer insulation system. They are often the earliest sign of insulation aging and can be present for extended periods before evolving into more severe faults. PD activity is typically detected at relatively low methane concentrations with minimal acetylene and ethylene.
D1 — Low-Energy Discharges (Sparking)
Low-energy electrical discharges, or sparking, typically result from poor electrical connections, floating potential components, or intermittent contact within the transformer. D1 faults produce moderate gas levels and may not cause immediate failure, but they warrant monitoring as they can escalate to high-energy discharges.
D2 — High-Energy Discharges (Arcing)
High-energy electrical discharges (arcing) generate significant volumes of fault gases, particularly acetylene. D2 faults represent sustained arcing that can rapidly degrade insulation and lead to catastrophic transformer failure if not addressed promptly. This is one of the most critical fault types to identify.
DT — Mixed Electrical & Thermal Faults
The DT (Discharge + Thermal) region represents a combination of electrical discharge activity and thermal decomposition. This mixed-fault zone indicates that both discharge and overheating mechanisms are contributing to the gas generation, making diagnosis more complex. Further investigation combining DGA with other diagnostic tests is recommended.
T1 — Thermal Fault < 300°C
Low-temperature thermal faults, typically from prolonged overloading, inadequate cooling, or minor insulation degradation. At temperatures below 300°C, the dominant fault gas is methane with relatively low ethylene production. T1 faults are generally less severe but may indicate developing problems.
T2 — Thermal Fault 300–700°C
Medium-temperature thermal faults often originate from localised hot spots in the transformer core or windings. These faults produce significant ethylene and moderate methane levels. T2 faults require attention as they can accelerate insulation aging and reduce transformer lifespan.
T3 — Thermal Fault > 700°C
High-temperature thermal faults, above 700°C, indicate severe overheating typically from sustained arcing, core circulating currents, or serious cooling failures. T3 faults produce high ethylene concentrations and require immediate investigation and corrective action to prevent permanent transformer damage.
How the Triangle Classifies Faults
Duval's Triangle uses the relative percentages of three gases (after normalising their sum to 100%). A data point's position on the ternary plot determines which fault region it belongs to. The region boundaries are defined by empirical thresholds established by Michel Duval through extensive analysis of transformer fault data.
Why Seven Regions?
The seven-region scheme (Triangle 1) is the most widely used version of Duval's Triangle. It provides sufficient resolution to distinguish between partial discharge, electrical, and thermal fault types while remaining simple enough for practical field use by DGA analysts and power engineers.